In order to provide desired preventative or diagnostic health care, a physician must often determine the level of various analytes in a patient's blood. For example, the level of glucose or cholesterol in the blood is often important in effective treatment of various diseases, e.g. diabetes, hypoglycemia, liver and thyroid disorders and atherosclerosis.
Generally, such analytes are measured in blood serum or plasma after the whole red blood cells have been removed. However, it would be desirable to be able to measure analytes in undiluted whole blood, thereby avoiding procedures required for separating red blood cells from the rest of the fluid and attendant labor and equipment costs. Using undiluted whole blood in analyses would also allow for simpler and faster sample procurement and processing. This would be especially useful for home monitoring assays wherein the assay procedure should be as simple as possible.
"Dry chemistry" assays are known. Such assays are analytical clinical techniques wherein chemical reagents are incorporated in various substantially "dry-to-the-touch" elements, e.g. test strips and multizone analytical elements. The advantages of "dry chemistry" assays over "wet chemistry" assays (i.e. techniques using reagents in solutions) are also known and include simplicity of use, economic savings and rapid analysis. However, analysis of whole blood using dry chemistry assays must overcome a serious problem. The corpuscular (red and white cells) and other high molecular weight components of whole blood must either be removed from the sample or somehow accommodated by the element in order to provide an accurate assay. State-of-the-art dry assays require removal of corpuscular components by allowing serum or plasma to penetrate an element and wiping off the corpuscular components which are unable to penetrate. Alternatively, for an element to accommodate the components, the void volume and pore size within the surface contacted by a blood sample must be sufficient to completely absorb the sample without clogging the analytical element. At the same time, the pore structure must not be so large as to cause mechanical instability (i.e. disintegration or fragmentation) of the element.
This problem is recognized in U.S. Pat. No. 4,312,834 (issued Jan. 26, 1982 to Vogel et al) wherein a monolayer diagnostic agent for analysis of fluids is disclosed. This diagnostic agent comprises a film forming material having a film opener therein to provide porosity. When the diagnostic agent is to be used for the detection of high molecular weight and corpuscular materials, the ratio of film opener to film former is higher than when low molecular weight materials are to be detected. However, the diagnostic agent taught by this reference has insufficient porosity to absorb whole blood cells as seen in Examples 4 and 5 wherein residual blood was wiped off the agent after 1 minute reaction time and prior to spectrophotometric measurement. The practice of wiping off residual corpuscular components prior to quantitative measurement of an analyte is common to commercially-available whole blood elements or diagnostic agents (see, e.g. U.K. No. 911,181, published Nov. 21, 1962).
Whole blood can also be assayed with an element which has a porous outer spreading layer which acts as a filter to trap the large corpuscular components of whole blood while allowing the serum or plasma to pass through to a separate reagent layer, which layer contains the necessary reagents for causing a detectable change to occur in the presence of a particular analyte. This technique is illustrated, for example, in Column 25 of U.S. Pat. No. 4,144,306 (issued Mar. 13, 1979 to Figueras), Example 4 of U.S. Pat. No. 4,258,001 (issued Mar. 24, 1981 to Pierce et al), Example 3 of U.S. Pat. No. 4,292,272 (issued Sept. 29, 1981 to Kitajima et al) and Example 2 of Japanese Patent Publication No. 57-101760 (published June 24, 1982).
However, it would be advantageous to avoid the need to separate the serum or plasma from corpuscular components in whole blood. This procedure of filtering the components from the serum or plasma using a filter layer is a slow process. There is also the likelihood that a portion of the analyte is lost in the filter/spreading layer as the plasma or serum passes through it, thereby resulting in an inaccurate analysis.
U.S. Pat. No. 4,042,335 (issued Aug. 16, 1977 to Clement) describes a whole blood assay utilizing a multilayer analytical element. The described whole blood element comprises a support having thereon, in order, a registration layer, a radiation-blocking layer and a reagent layer. The reagent layer can act as a porous spreading layer while the radiation-blocking layer can act as a filter layer to filter out and exclude whole blood cells from the registration layer (see FIG. 1 of the reference), thereby avoiding interference by hemoglobin.
While such an element may be used to determine an analyte in whole blood, its usefulness depends to a large degree on having a detectable species which can rapidly diffuse through the radiation-blocking layer to the registration layer and which has a sufficiently high extinction coefficient. However, not all detectable species (e.g. dyes, color-forming couplers, etc.) satisfy these requirements.
Hence, it would be desirable to have a simple and rapid "dry chemistry" assay for undiluted whole blood which provides rapid and accurate analysis and eliminates the need to wipe off residual blood.